1. Technical Field
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and the like, and in particular to an image forming apparatus employing electrophotography to form images.
2. Related Art
In image forming apparatuses employing electrophotography, a surface of an image carrier such as a photoreceptor is uniformly charged by a charger; an electrostatic latent image is formed on the surface of the image carrier by an exposure device; and a developing device adheres toner onto the electrostatic latent image to thus form a toner image. Such image forming apparatuses are widely used not only in the business offices but in industrial printing because of ease of draft generation and correction and acute demand for ever higher output speed and quality.
Of those various quality requirements, uniform density over any given printed page is highly demanded and the uniformity in the printed page is a decision factor when a user selects an image forming apparatus. Accordingly, suppressing density fluctuation in the printed page is most important. It is known that density fluctuation occurs due to various factors. Those factors include, for example, uneven charging or charging fluctuation when the charger electrically charges the surface of the image carrier; fluctuation of exposure by the exposure unit; eccentric rotation and variations in sensitivity of an image carrier such as a photoreceptor; eccentric rotation or variations in the resistance of a developer carrier such as a developing roller; fluctuation in the charge of the toner; and variations in the transferring of a transfer roller.
Among those factors, eccentric rotation or sensitivity fluctuation of the image carrier (being a rotary member) in the rotational direction, and eccentric rotation or variations in the electrical resistance of the developer carrier (being a rotary member) in the rotational direction are prominent. In general, the image carrier and the developer carrier are rotated more than once to form a toner image to be formed in one image, i.e., one page image. Thus, a cyclic image density fluctuation is generated due to the above factors, and the image density fluctuation appearing in the formed image is easily apparent. Therefore, minimizing image density fluctuation is paramount.
JP-H09-062042-A discloses an image forming apparatus in which cyclic density fluctuation data is stored for the purpose of exclusively reducing the stripe-shaped density fluctuation generated cyclically in the output image and image forming conditions are adjusted based on the density fluctuations data. According to this image forming apparatus, the density fluctuations data (i.e., the image density fluctuation data) corresponding to at least one rotary cycle of the developer carrier is stored, and any one of charge voltage, exposure light amount, developing voltage, and transfer voltage is adjusted to reduce the image density fluctuation corresponding to the density fluctuations data. Similarly, JP-2000-98675-A discloses an image forming apparatus in which the image density fluctuation of the developer carrier having a rotation cycle is reduced by adjusting image processing conditions in accordance with one rotary cycle of the developer carrier.
The mechanism by which image density fluctuates due to the eccentric rotation of the image carrier or the developer carrier will be described in detail using the eccentric rotation of the image carrier as an example.
An electric potential difference is created between the image carrier and the developer carrier disposed opposite and in the vicinity of the image carrier and a developing bias is applied to the developing area, so that an electric field is generated in the developing area. Toner is transferred onto an electrostatic image formed on the surface of the image carrier by the electric field and is adhered thereon to form a toner image. If the image carrier suffers from eccentric rotation, it gets out of synch with the developer carrier. Thus, even though the developing bias is kept constant, the electric field strength in the developing area fluctuates with the rotary cycle of the image carrier. Because a toner deposition amount per unit area adhered onto the electrostatic latent image changes relative to the electric field strength in the developing area, if the image carrier does not rotate at a constant speed, the toner deposition amount per unit area changes in accordance with the rotary cycle of the image carrier even though the same image density is to be obtained. The same applies to the eccentric rotation of the developer carrier.
In addition, the sensitivity fluctuation of the image carrier in the rotary direction of the image carrier changes the potential of the electrostatic latent image portion on the surface of the image carrier, so that the potential difference between the electrostatic latent image portion on the surface of the image carrier and the developer carrier changes during the rotary cycle of the image carrier. As a result, when the sensitivity fluctuation exists in the rotary direction of the image carrier, the toner deposition amount per unit area to be adhered on the electrostatic latent image changes even though the same image density is to be obtained. The same applies to the variations in the resistance of the developer carrier in the rotary direction of the developer carrier.